Solution spun fiber process

ABSTRACT

The invention relates to a process for forming fibers from a spinning solution utilizing a high speed rotary sprayer. The fibers can be collected into a uniform web for selective barrier end uses. Fibers with an average fiber diameter of less that 1,000 nm can be produced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for forming fibers and fibrous webs.In particular, very fine fibers can be made and collected into a fibrousweb useful for selective barrier end uses such as filters, batteryseparators, and breathable medical gowns.

2. Background of the Invention

Rotary sprayers used in conjunction with a shaping fluid and anelectrical field are useful in atomizing paint for coating a targetdevice. The centrifugal force supplied by the rotary sprayers producesenough shear to cause the paint to become atomized and the shaping fluidand electrical field draw the atomized paint to the target device. Thisprocess has been optimized for the production of atomized droplets.Defects occur when too many atomized droplets agglomerate into largerentities. The prior art teaches toward making atomized droplets and notlarger entities.

There is a growing need for very fine fibers and fibrous webs made fromvery fine fibers. These types of webs are useful for selective barrierend uses. Presently very fine fibers are made from melt spun “islands inthe sea” cross section fibers, split films, some meltblown processes,and electrospinning. What is needed is a high throughput process to makevery fine fibers and uniform fibrous webs.

SUMMARY OF THE INVENTION

The present invention provides a high throughput process to make veryfine fibers and uniform webs by the use of a high speed rotary sprayer.

In a first embodiment, the present invention is directed to a fiberforming process comprising the steps of supplying a spinning solutionhaving at least one polymer dissolved in at least one solvent to arotary sprayer having a rotating conical nozzle, the nozzle having aconcave inner surface and a forward surface discharge edge; issuing thespinning solution from the rotary sprayer along the concave innersurface so as to distribute said spinning solution toward the forwardsurface of the discharge edge of the nozzle; and forming separatefibrous streams from the spinning solution while the solvent vaporizesto produce polymeric fibers in the absence of an electrical field. Ashaping fluid can flow around the nozzle to direct the spinning solutionaway from the rotary sprayer. The fibers can be collected onto acollector to form a fibrous web.

In a second embodiment, the present invention is directed to a fiberforming process comprising the steps of supplying a spinning solutionhaving at least one polymer dissolved in at least one solvent to arotary sprayer having a rotating conical nozzle, the nozzle having aconcave inner surface and a forward surface discharge edge; issuing thespinning solution from the rotary sprayer along the concave innersurface so as to distribute said spinning solution toward the forwardsurface of the discharge edge of the nozzle; and forming separatefibrous streams from the spinning solution while the solvent vaporizesto produce polymeric fibers in the presence of an electrical field. Ashaping fluid can flow around the nozzle to direct the spinning solutionaway from the rotary sprayer. The fibers can be collected onto acollector to form a fibrous web.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a nozzle portion of a rotary sprayer forforming fibers suitable for use in the present invention.

FIG. 2 a is a scanning electron micrograph of poly(ethylene oxide)fibers made without an electrical field according to the process of thepresent invention.

FIG. 2 b is a scanning electron micrograph of the fibers of FIG. 2 a asthey were distributed onto a collection scrim.

FIG. 3 a is a scanning electron micrograph of poly(ethylene oxide)fibers made with an electrical field according to the process of thepresent invention.

FIG. 3 b is a scanning electron micrograph of the fibers of FIG. 2 a asthey were distributed onto a collection scrim.

FIG. 4 is a scanning electron micrograph of poly(vinyl alcohol) fibersmade with an electrical field according to the process of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a process for forming fibers from a spinningsolution utilizing a rotary sprayer.

The spinning solution comprises at least one polymer dissolved in atleast one solvent. Any fiber forming polymer able to dissolve in asolvent that can be vaporized can be used. Suitable polymers includepolyalkylene oxides, poly(meth)acrylates, polystyrene based polymers andcopolymers, vinyl polymers and copolymers, fluoropolymers, polyestersand copolyesters, polyurethanes, polyalkylenes, polyamides, polyaramids,thermoplastic polymers, liquid crystal polymers, engineering polymers,biodegradable polymers, bio-based polymers, natural polymers, andprotein polymers. The spinning solution can have a polymer concentrationof about 1% to about 90% by weight of polymer in the spinning solution.Also, in order to assist the spinning of the spinning solution, thespinning solution can be heated or cooled. Generally, a spinningsolution with a viscosity from about 10 cP to about 100,000 cP isuseful.

FIG. 1 is an illustration of a nozzle portion of a rotary sprayer 10suitable for forming fibers from the spinning solution. A spinningsolution is prepared by dissolving one or more polymers in one or moresolvents. The spinning solution is pumped through a supply tube 20running axially through the rotary sprayer 10. The throughput rate ofthe solution is from about 1 cc/min to about 500 cc/min. As the spinningsolution exits the supply tube 20 it is directed into contact with arotating conical nozzle 30 and travels along the nozzle's concave innersurface 32 until it reaches the nozzle's forward surface discharge edge34. A rotational speed of conical nozzle 30 is between about 10,000 rpmand about 100,000 rpm. The conical nozzle 30 can be any conical-likeshape having a generally concave inner surface, including a bell shapesuch as illustrated here, a cup shape or even a frusto-conical shape.The shape of the nozzle's concave inner surface 32 can influence theproduction of fibers. The cross section of the nozzle's concave innersurface 32 can be straight or curved. The shape of the nozzle's forwardsurface discharge edge 34 can also influence the production of fibers.The nozzle's forward surface discharge edge 34 can be sharp or roundedand can include serrations or dividing ridges. Optionally, a distributordisk 40 can be used to help direct the spinning solution from the supplytube 20 to the inner concave surface 32 of nozzle 30. The rotation speedof the nozzle propels the spinning solution along the nozzle's concaveinner surface 32 and past the nozzle's forward surface discharge edge 34to form separate fibrous streams, which are thrown off the dischargeedge by centrifugal force. Simultaneously, the solvent vaporizes untilfibers of the invention are formed. The fibers can be collected on acollector (not shown) to form a fibrous web.

Optionally, FIG. 1 shows shaping fluid housing 50 which guides shapingfluid (marked by arrows) around nozzle 30 to direct the spinningsolution away from the rotary sprayer 10. The shaping fluid can be agas. Various gases and at various temperatures can be used to decreaseor to increase the rate of solvent vaporization to affect the type offiber that is produced. Thus, the shaping gas can be heated or cooled inorder to optimize the rate of solvent vaporization. A suitable gas touse is air, but any other gas which does not detrimentally affect theformation of fibers can be used.

Optionally, an electrical field can be added to the process. A voltagepotential can be added between the rotary sprayer and the collector.Either the rotary sprayer or the collector can be charged with the othercomponent substantially grounded or they can both be charged so long asa voltage potential exists between them. In addition, an electrode canbe positioned between the rotary sprayer and the collector wherein theelectrode is charged so that a voltage potential is created between theelectrode and the rotary sprayer and/or the collector. The electricalfield has a voltage potential of about 1 kV to about 150 kV.Surprisingly, the electrical field seems to have little effect on theaverage fiber diameter, but does help the fibers to separate and traveltoward a collector so as to produce a more uniform fibrous web.

This process can make very fine fibers, preferably continuous fibers,with an average fiber diameter of less than 1,000 nm and more preferablyfrom about 100 nm to 500 nm. The fibers can be collected on a collectorinto a fibrous web. The collector can be conductive for creating anelectrical field between it and the rotary sprayer or an electrode. Thecollector can also be porous to allow the use of a vacuum device to pullvaporized solvent and optionally shaping gas away from the fibers andhelp pin the fibers to the collector to make the fibrous web. A scrimmaterial can be placed on the collector to collect the fiber directlyonto the scrim thereby making a composite material. For example, aspunbond nonwoven can be placed on the collector and the fiber depositedonto the spunbond nonwoven. In this way composite nonwoven materials canbe produced.

Test Methods

In the description above and in the non-limiting examples that follow,the following test methods were employed to determine various reportedcharacteristics and properties.

Viscosity was measured on a Thermo RheoStress 600 rheometer equippedwith a 20 mm parallel plate. Data was collected over 4 minutes with acontinuous shear rate ramp from 0 to 1,000 s⁻¹ at 23° C. and reported incP at 10 s⁻¹.

Fiber Diameter was determined as follows. Ten scanning electronmicroscope (SEM) images at 5,000× magnification were taken of eachnanofiber layer sample. The diameter of eleven (11) clearlydistinguishable nanofibers were measured from each SEM image andrecorded. Defects were not included (i.e., lumps of nanofibers, polymerdrops, intersections of nanofibers). The average fiber diameter for eachsample was calculated and reported in nanometers (nm).

EXAMPLES

Hereinafter the present invention will be described in more detail inthe following examples.

Example 1 describes making a poly(ethylene oxide) continuous fiberwithout the use of an electrical field. Example 2 describes making apoly(ethylene oxide) continuous fiber with the use of an electricalfield. Example 3 describes making a poly(vinyl alcohol) continuous fiberwith the use of an electrical field.

Example 1

Continuous fibers were made using a standard Aerobell rotary atomizerand control enclosure for high voltage, turbine speed and shaping aircontrol from ITW Automotive Finishing Group. The bell-shaped nozzle usedwas an ITW Ransburg part no. LRPM4001-02. A spinning solution of 10.0%poly(ethylene oxide) viscosity average molecular weight (Mv) of about300,000, 0.1% sodium chloride, and 89.9% water by weight was mixed untilhomogeneous and poured into a Binks 83C-220 pressure tank for deliveryto the rotary atomizer through the supply tube. The pressure on thepressure tank was set to a constant 15 psi. This produced a flow rate ofabout 2 cc/min. The shaping air was set at a constant 30 psi. Thebearing air was set at a constant 95 psi. The turbine speed was set to aconstant 40,000 rpm. No electrical field was used during this test.Fibers were collected on a Reemay nonwoven collection screen that washeld in place 10 inches away from the bell-shaped nozzle by stainlesssteel sheet metal. The fiber size was measured from an image usingscanning electron microscopy (SEM) and determined to be in the range of100 nm to 500 nm, with an average fiber diameter of about 415 nm. An SEMimage of the fibers can be seen in FIG. 2 a. FIG. 2 b is a SEM imagewhich shows the distribution of the fibers spun according to thisExample on the Reemay scrim.

Example 2

Example 2 was prepared similarly to Example 1, except an electricalfield was applied. The electrical field was applied directly to therotary atomizer by attaching a high voltage cable to the high voltagelug on the back of the rotary atomizer. The rotary atomizer wascompletely isolated from ground using a large Teflon stand so that theclosest ground to the bell-shaped nozzle was the stainless steel sheetmetal backing the Reemay collection belt. A +50 kV power supply was usedin current control mode and the current was set to 0.02 mA. The highvoltage ran at about 35 kV. The lay down of the fiber was much betterthan in Example 1 in that the coverage was very uniform over thecollection area. The fiber size was measured from an image using SEM anddetermined to be in the range of 100 nm to 500 nm, with an average fiberdiameter of about 350 nm. An SEM image of the fibers can be seen in FIG.3 a. FIG. 3 b is a SEM image which shows the distribution of the fibersspun according to this Example on the Reemay scrim.

Example 3

Continuous fibers were made using a 65 mm “Eco Bell” serratedbell-shaped nozzle on a Behr rotary atomizer. A spin solution of 15%Evanol 80-18 poly(vinyl alcohol) and water by weight was mixed untilhomogeneous and poured into a pressure tank for delivery to the rotaryatomizer through the supply tube. The viscosity of the spinning solutionwas 2,000 cP at 23° C. The pressure on the pressure tank was set to aconstant pressure so that the flow rate was measured to be 17 cc/min.The shaping air was set at 100 SL/min. The turbine speed was set to aconstant 50,000 rpm. An electrical field was applied directly to therotary atomizer and the high voltage was set to 50 kV. Fibers werecollected on a spunbond/meltblown/spunbond (SMS) composite nonwovencollection screen that was held in place 21 inches away from thebell-shaped nozzle by grounded stainless steel sheet metal. The fibersize was measured from an image using SEM and determined to be in therange of 100 nm to 600 nm with an average fiber diameter of 415 nm. SEMimage of the fibers can be seen in FIG. 4.

1. A fiber forming process comprising the steps of: supplying a spinningsolution having at least one polymer dissolved in at least one solventto a rotary sprayer having a rotating conical nozzle, the nozzle havinga curved concave inner surface and a forward surface discharge edge;issuing the spinning solution from the rotary sprayer along the curvedconcave inner surface so as to distribute said spinning solution towardthe forward surface of the discharge edge of the nozzle; and formingseparate fibrous streams which are thrown off the discharge edge bycentrifugal force from the spinning solution while the solvent vaporizesto produce continuous polymeric fibers in the absence of an electricalfield, wherein the fibers have an average fiber diameter of less thanabout 1,000 nm; wherein the nozzle further comprises a distributor diskfor directing the spinning solution towards the concave inner surface ofthe nozzle.
 2. The process of claim 1, wherein the polymer is selectedfrom the group comprising polyalkylene oxides, poly(meth)acrylates,polystyrene based polymers and copolymers, vinyl polymers andcopolymers, fluoropolymers, polyesters and copolyesters, polyurethanes,polyalkylenes, polyamides, polyaramids, thermoplastic polymers, liquidcrystal polymers, engineering polymers, biodegradable polymers,bio-based polymers, natural polymers, and protein polymers.
 3. Theprocess of claim 1, wherein the spinning solution has a concentration ofpolymer dissolved in solvent of about 1% by weight of polymer to about90% by weight of polymer.
 4. The process of claim 1, wherein thespinning solution can be heated or cooled.
 5. The process of claim 1,wherein the spinning solution has a viscosity from about 10 cP to about100,000 cP.
 6. The process of claim 1, wherein the spinning solution issupplied at a throughput rate from about 1 cc/min to about 500 cc/min.7. The process of claim 1, wherein the rotational speed of the nozzle isbetween about 10,000 rpm and about 100,000 rpm.
 8. The process of claim1, wherein the average fiber diameter is about 100 nm to about 500 nm.9. The process of claim 1, further comprising flowing a shaping fluidaround the nozzle to direct the spinning solution away from the rotarysprayer.
 10. The process of claim 9, wherein the shaping fluid comprisesa gas.
 11. The process of claim 10, wherein the gas is air.
 12. Theprocess of claim 1, further comprising collecting the fiber onto acollector to form a fibrous web.
 13. The process of claim 12, furthercomprising applying a vacuum through the collector to pull the fibersonto the collector to form a fibrous web